In the field of waste management, there is a need for metallic materials which resist hot, strong acids and hot, strong caustic alkalis. This is because such chemicals are used to neutralize one another, resulting in more stable and less hazardous compounds. Of the acids used in industry, sulfuric is the most important in terms of the quantities produced. Of the caustic alkalis, sodium hydroxide (caustic soda) is the most commonly used.
Certain nickel alloys are very resistant to strong, hot sulfuric acid. Others are very resistant to hot, strong sodium hydroxide. However, none possesses adequate resistance to both chemicals.
Typically, nickel alloys with high alloy contents are used to resist sulfuric acid and other strong acids, the most resistant being the nickel-molybdenum and nickel-chromium-molybdenum alloys.
On the other hand, pure nickel (UNS N02200/Alloy 200) or nickel alloys with low alloy contents are the most resistant to sodium hydroxide. Where higher strength is required, the nickel-copper and nickel-chromium alloys are used. In particular, alloys 400 (Ni—Cu, UNS N04400) and 600 (Ni—Cr, UNS N06600) possess good resistance to corrosion in sodium hydroxide.
During the discovery of the alloys of this invention, two key environments were used, namely 70 wt. % sulfuric acid at 93° C. (200° F.) and 50 wt. % sodium hydroxide at 121° C. (250° F.). 70 wt. % sulfuric acid is well known to be very corrosive to metallic materials, and is the concentration at which the resistance of many materials (including the nickel-copper alloys) breaks down, as a result of changes in the cathodic reaction (from reducing to oxidizing). 50 wt. % sodium hydroxide is the concentration most widely used in industry. A higher temperature was used in the case of sodium hydroxide to increase internal attack (the main form of degradation of nickel alloys in this chemical), hence increase the accuracy of measurements during subsequent cross-sectioning and metallographic examination.
In U.S. Pat. No. 6,764,646 Crook et al. describe nickel-chromium-molybdenum-copper alloys resistant to sulfuric acid and wet process phosphoric acid. These alloys require copper in the range 1.6 to 2.9 wt. %, which is below the levels required for resistance to 70% sulfuric acid at 93° C. and 50% sodium hydroxide at 121° C.
U.S. Pat. No. 6,280,540 to Crook discloses copper-containing, nickel-chromium-molybdenum alloys which have been commercialized as C-2000® alloy and correspond to UNS 06200. These contain higher molybdenum levels and lower chromium levels than in the alloys of this invention and lack the aforementioned corrosion characteristics.
U.S. Pat. No. 6,623,869 to Nishiyama et al. describes nickel-chromium-copper alloys for metal dusting service at high temperatures, the maximum copper contents of which are 3 wt. %. This is below the range required for resistance to 70% sulfuric acid at 93° C. and 50% sodium hydroxide at 121° C. More recent U.S. Patent Application Publications (US 2008/0279716 and US 2010/0034690) by Nishiyama et al. describe additional alloys for resistance to metal dusting and carburization. The alloys of US 2008/0279716 differ from the alloys of this invention in that they have a molybdenum restriction of not more than 3%. The alloys of US 2010/0034690 are in a different class, being iron-based, rather than nickel-based, with a molybdenum content of 2.5% or less. U.S. Published Patent Application No. US2011/0236252 to Ueyama et al. discloses nickel-chromium-molybdenum-copper alloys resistant to reducing hydrochloric and sulfuric acids. The given range in these alloys for chromium is 20 to 30% and for copper it is 2 to 5%; however, the inventive alloy examples given in this patent contain chromium up to 23% and copper up to 3.06%, which are below the levels needed for resistance to 70% sulfuric at 93° C. and 50% sodium hydroxide at 121° C.